1. Field of the Invention
This invention relates generally to the measurement of a property of a layer in a multilayered structure, and in particular to a measurement of reflectance of a region of a conductive layer (such as a conductive line), and use of the reflectance measurement to determine various properties, such as sheet resistance of the layer, and thermal conductivity of a dielectric layer located underneath the conductive layer.
2. Description of Related Art
Metal lines having sub-micron (i.e. less than 1 micron) dimensions are conventionally used to interconnect devices that are formed in an integrated circuit die. Such a metal line is typically formed as a portion of a film of metal (such as aluminum or copper). The metal film is normally formed as a blanket layer over a semiconductor wafer, and is thereafter removed (e.g. by etching) to form one or more metal lines, in a process act known as "patterning". Conventionally, the resistivity of the metal film is measured (on a test wafer), and the measurement is combined with a measurement of the film's thickness (on another test wafer) and, a measurement of the line width (on a production wafer), to determine if the metal film has ohmic loss sufficiently low for use in forming metal lines required in an integrated circuit die.
A number of methods exist for measuring a metal film's resistivity. Two such methods are commonly known as "probing" and "eddy current". In the probing method, two or four probes are brought into physical contact with an unpatterned metal film (e.g. on a test wafer) to measure the film's resistivity directly. See, for example, "The Four-Point Probe", Section 1.2, pages 2-20 in the book "Semiconductor Material and Device Characterization" by Dieter K. Schroder, John Wiley & Sons, Inc, New York, 1990. In the eddy current method, a measurement device is coupled to the metal film either capacitively or inductively, i.e. without contacting the metal film. See, for example, "Eddy Current", Section 1.4.1, pages 27-30, in the book by Schroder (referenced above).
Each of the above-described methods requires a metallized region having a width (e.g. 0.5 mm) that may be several orders of magnitude larger than a typical metal line's width (e.g. &lt;0.5 microns). Due to the requirement of the metallized region to have a 1000 times larger width, the measurements are performed prior to patterning, typically on a test wafer. Moreover, the above-described methods measure merely the resistivity of a metal film, and are not known to be used in the measurement of resistance of a line formed after etching the metal film (e.g. in a production wafer).
U.S. Pat. No. 5,228,776 granted to Smith et al. (hereinafter "Smith") describes measuring changes in optical reflectivity (column 4, line 5-6) caused by thermal waves (column 3, line 42) to "monitor variations in electrical conductivity and resistance . . . " (column 4, lines 53-54). Specifically, Smith requires "periodically exciting the sample at a highly localized spot on the sample surface . . . The pump beam functions to periodically heat the sample which in turn generates thermal waves that propagate from the irradiated spot . . . Features at or beneath the sample surface can be studied by monitoring the variations they induce in these waves" (column 1, lines 25-40). Smith also states that "when the optical reflectivity of the sample is to be monitored, it is desirable to arrange the pump and probe beams to be coincident on the sample" (column 1, lines 60-64). When using such coincident beams, Smith notes problems created by "surfaces associated with defective vias are often not optically flat . . . " (column 3, lines 6-13). Moreover, prior art also states that "[w]hen materials other than semiconductors are to be evaluated, such as metals . . . analysis of the thermal wave patterns is required" (see U.S. Pat. No. 4,854,710 at column 7, lines 41-44).